Flexographic printing that is used for packaging materials such as corrugated boards, paper containers, paper bags and flexible packaging films, building and decoration materials such as wallpapers and decorative plates, label printing and the like has an increased proportion among a various kinds of printing processes. For fabrication of printing plates for use in the printing processes, normally, photosensitive resins are often used. For example, a process in which a liquid photosensitive resin or a solid photosensitive resin plate in the form of a sheet is used, a photo mask is placed on the photosensitive resin, light is applied through the mask to cause a crosslinking reaction, and non-crosslinked parts are then washed out with a developing solution has been used. In recent years, so called a flexographic CTP (Computer to Plate) technique of providing a thin light absorption layer called a black layer on the surface of a photosensitive resin, applying laser light to the layer to form a mask image directly on a photosensitive resin plate, then applying light through the mask to cause a crosslinking reaction, and then washing out non-crosslinked parts of areas to which no light is applied has been developed and increasingly employed in terms of the effect of improving efficiency of fabrication of a printing plate. However, this technique has a limited effect of improving the efficiency, and has a problem of absolutely requiring a developing step, and a technique in which a relief image is formed directly on a printing original plate and no developing step is required is desired.
A process for solving the problem is a process in which a printing original plate is directly subjected to laser engraving. Fabrication of letterpress printing plates and stamps by this process has already been conducted. As materials for use in this process, materials prepared by heating and curing synthetic rubbers such as EPDM and silicones have already been used. However, not only these materials require much time for production because it takes much time for heating and curing to achieve necessary mechanical properties, aging is further required until the properties become stable, and so on, but also the former materials using synthetic resins and the like as raw materials have a disadvantage that sculpture debris is stuck on the surface of a plate when the plate is subjected to laser engraving, and it is very difficult to remove the debris, and the latter materials using silicones as raw materials have a disadvantage that the speed of laser engraving is so low that it takes much time to fabricate a plate, the resistance to a solvent ink is low, and so on.
As a process for overcoming the disadvantages of the materials described above, a process for producing a laser engraved flexographic printing plate, in which laser light is applied to a cured photosensitive resin obtained by photo-curing a photosensitive resin composition to form an irregular pattern on the surface, has been proposed.
For example, Patent Document 1 (Japanese Patent No. 2846954 (U.S. Pat. No. 5,798,202)) and Patent Document 2 (Japanese Patent No. 2846955 (U.S. Pat. No. 5,804,353)) disclose use of a material prepared by mechanically, photochemically and thermochemically reinforcing a thermoplastic elastomer such as SBS, SIS or SEBS.
Patent Document 3 (JP-A-56-64823) discloses the use of roll material prepared by photo-curing a liquid photosensitive resin. Further, the present inventors have proposed in Patent Document 4 (WO 03-022594 Pamphlet) a laser engraved printing original plate using a liquid photosensitive resin composition containing a polymeric material that is a plastomer at 20° C., and proposed a process allowing improvements in which an inorganic porous material is made to coexist to inhibit generation of liquid debris caused by a laser, tacks on the surface of the plate are reduced, and an optical system is prevented from being contaminated.
Further, in recent years, demands for printing of continuous patterns have been increased, and in this application, development of a cylindrical printing substrate such as a seamless sleeve having no seams, which is easily subjected to laser engraving and capable of high quality printing, has come to be highly desired.
As described previously, the laser engraved original plate using a cured photosensitive resin has a good laser engraving characteristic as compared to vulcanized synthetic rubbers and silicone rubbers, and it is expected that the edge of an image will be so sharp that fine patterns can be formed, thus making it possible to meet high quality printing, but it is not sufficient. As a precondition for this, it is extremely important to ensure the accuracy of the thickness of the printing plate, and printing quality highly depends on the plate thickness accuracy before a laser engraving step. In the case of a laser engravable printing substrate, high film thickness accuracy can be ensured by subjecting the surface of a cured photosensitive resin obtained via a photo-curing step to surface processing such as cutting, grinding and polishing. Particularly, in the case of a cylindrical printing substrate such as a seamless sleeve, it can be placed in a printer to perform printing just after a pattern is formed by a laser engraving process, and therefore ensuring the plate thickness accuracy is an extremely important step in a process of fabricating a laser engravable printing substrate. Of course, the plate thickness accuracy is also important when a flat printing plate is wound around a cylinder of a printer to perform printing, and the same holds true for a sheeted original plate for laser engraving.
Patent Document 1 (Japanese Patent No. 2846954 (U.S. Pat. No. 5,798,202)) and Patent Document 2 (Japanese Patent No. 2846955 (U.S. Pat. No. 5,804,353) describes a process in which a photosensitive resin composition molded in the form of a sheet is cylindrically wound, and joint areas are melted to be fused to achieve seamlessness. These documents also describe that fused areas can be polished. The present inventors have proposed in Patent Document 5 (PCT/JP 2004/005839) a process for producing a seamless cylindrical printing substrate using a liquid photosensitive resin, and proposed that surface processing such as cutting, grinding and polishing is carried out after curing by light.
The present inventors have examined surface processing processes such as cutting, grinding and polishing further in detail using the conventional photosensitive resin composition described previously, and as a result, it has been found that since sticky debris generated in these processes is deposited on the surface, and deposited sticky debris is hard to be removed, cutting and polishing marks may remain on the surface, and the processing accuracy of the obtained surface becomes so insufficient that a problem arises in printing quality especially when performing precise printing. The above described sticky debris is deposited on the surface of a cutting bite or a grinding wheel, and clings in some cases. It has been found that for avoiding these problems, carefully performing processing, such as stopping working to clean out or remove deposited debris when the sticky debris clings to the cutting bite or the grinding wheel, is required, and therefore a considerable amount of time is required for processing.
The process proposed by the present inventors in the aforementioned Patent Documents 3 and 5 in which a liquid photosensitive resin composition is used at 20° C. is a quite excellent process in that owing to a characteristic of a liquid form, the photosensitive resin composition is easily molded and can adapt to either a sheet form or sleeve form by a simple process such as coating using a blade, but it has been newly found that due to limitations on a resin design for liquefying a resin curing, a printing plate sculptured with a laser after curing has a problem of being prone to occurrence of “ink bridging” in which an ink remains in areas between formed relief patterns to degrade printing quality.
There are several proposals as conventional techniques for improving the surface characteristic of a photosensitive resin plate for printing although they are not techniques relating to a laser engravable printing substrate. For example, Patent Document 6 (JP-A-6-186740) describes a printing substrate capable of aqueous development for forming irregular patterns on the surface using a photoengraving technique, and describes that to a photosensitive resin is added a silicone compound having a polymerizable unsaturated group copolymerizing with the resin. As an effect of adding the above described silicone compound, inhibition of deposition of a dry ink and a paper powder on areas between formed relief patterns is described. However, Patent Document 6 relates to production of a printing plate using a photoengraving technique, but does not relate to a laser engravable printing substrate. If the silicone compound added for the purpose of control of wettability of the surface of the printing plate which is used in this patent document has a large number of polymerizable unsaturated groups, there is the concern that the density of crosslinking points increases to increase the hardness of the cured photosensitive resin or increase a photo-curing contraction coefficient. The present inventors examined “ink bridging” using the silicone compound described in Patent Document 6, resultantly found that since the silicone compound is easily transferred to the surface of the cured photosensitive resin and fixed on the surface during photo-curing, the surface is made excessively hydrophobic, and therefore rejects a solvent such as alcohol which is widely used in a solvent ink or the like, and recognized this as a problem in ensuring high printing quality.
A process in which the surface of a flexographic printing plate using a photosensitive resin is treated has also been proposed. Studies are conducted for the purpose of inhibiting a phenomenon in which an ink deposited on the surface of the plate during printing remains, and cannot be removed as it is trapped between fine patterns such as dots, thin letters and thin lines, and the remaining ink is transferred to printed matter to cause ink stains on non-image areas. This phenomenon tends to occur particularly in the case of printing over an extended period of time or if a pressure applied between the anilox roll and the plate is high when transferring an ink to the plate surface, and ink stains on non-image areas raise a serious problem in terms of printing quality. If such ink stains occur, a printer should be stopped to clean the plate surface, and productivity at a printing site is thus considerably impaired. A process in which a mixture of an aqueous emulsion of a silicone compound or a fluorine compound and an aqueous resin is coated is proposed in Patent Document 7 (JP-A-2002-292985), but the effect on the remaining of an ink on the plate surface is not necessarily sufficient because an aqueous solution having a low seepage force is coated. There is also a problem as to the persistence of the effect.
Patent Document 8 (JP-A-60-191238) describes an image reproduction material having a photosensitive resin layer, a scratch resistance layer and a protective layer, and describes that a silicone oil is included in the photosensitive resin layer and the silicone oil is transferred to the surface of the photosensitive resin layer to form a scratch layer. Patent Document 8 describes as one of applications a photosensitive resin letterpress on which irregular patterns are formed via light exposure and development steps, but this is not a laser engraving plate on which patterns are formed using the laser engraving process. For the added silicone oil, a compound that is easily transferred from the inside of the resin to the surface is selected, and therefore it is presumed that the silicone oil is not fixed in the resin. Thus, when the above described image forming material is used as a printing plate, there is a problem such that the silicone oil is extracted with a solvent in an ink used, and it is therefore difficult to maintain the scratch resistance effect for a long time.
Thus, the present inventors have recognized as two serious problems the problem as to removal of sticky debris generated in surface processing steps such as cutting, grinding and polishing in a step of producing a laser engravable printing substrate, which is composed of a cured photosensitive resin, and the problem of “ink bridging” between relief patterns of the laser engravable printing substrate. There is not a conventional technique for which a process capable of solving these two serious problems at the same time is described.
Patent Document 1: Japanese Patent No. 2846954
Patent Document 2: Japanese Patent No. 2846955
Patent Document 3: JP-A-56-64823
Patent Document 4: WO 03/022594 A
Patent Document 5: PCT/JP2004/005839
Patent Document 6: JP-A-6-186740
Patent Document 7: JP-A-2002-292985
Patent Document 8: JP-A-60-191238